An invention for detecting a cetane number as a property of a fuel that is used in a compression ignition engine has been proposed in a patent document 1 (i.e., Japanese Patent Laid-Open No. 2009-144528). The invention in the patent document 1 detects a cetane number of a fuel based on a torque change amount that corresponds to a change of a fuel injection amount in one of many sub-injections performed after a main injection, which is designated as a specific injection.
However, the fuel property is not only determined by a cetane number but also by a density, which affects a state of the internal combustion. That is, even when the two kinds of fuel have the same cetane number, the density difference of the two fuels results in respectively different combustion states. Therefore, for the optimization of the combustion, the combustion control must take into account the density of the fuel in addition to the cetane value. For such purposes, detecting both of the cetane number and the density of the fuel is beneficial. Further, the fuel density may be detected by using a separate and dedicated fuel density sensor. However, a separate and dedicated fuel density sensor leads to an increase of the number of sensors.